Apparatus and method to comminute solid particles in gas



H. G. TANNER May 15, 1951 APPARATUS AND METHOD TO COMMINUTE SOLIDPARTICLES IN GAS 2 Sheets-Sheet 1 Filed Aug. 27, 1948 Shiv Herbert G.Tunnr,

awk/ 1% ATTOR N EY H. e. TANNER 2,552,603

APPARATUS AND METHOD TO COMMINUTE SOLID PARTICLES IN GAS May.15, 1951 2Sheets-Sheet 2 Filed Aug. 27, 1948 FIG. 2

FIG. 3

INVENTOR HERBER'IY G. TANNER,

ATTORNEY Patented May 15, 1951 UNITED STATES PATENT OFLFHCE.

APPARATUS AND METHOD TO COMMINU'IE SOLID PARTICLES IN GAS (Granted underthe act of March 3, 1883, as amended April 30, 1928; 370 O. G. 757) 7Claims.

The invention described herein may be manufactured and used by or forthe Government, for governmental purposes, without the payment to me ofany royalty thereon.

This invention deals with comminution of finely divided solid particlesby means of highly turbulent gas. It relates to improved apparatus andmethods for applying such particles to turbulent gas to break them intoextremely fine dusts.

Heretofore currents of air have been used quite commonly to separate orclassify particles of one size or weight from another and even to grindthe particles to some extent. Jets of air in prior grinding mills orcolloid mills rub solid particles against each other or else throw theparticles against abutting elements of the grinder. Often the gasrecircuclates the material to be ground as it is blown around in theapparatus. This recirculation indicates defects such as inefficientgrinding and such as classification or great variation in particle size.Gas currents become less effective as particle size decreases and aslightness of the material increases. The use of gas in burdensomequantities has been heretofore necessary to obtain notable reduction insize.

Such dilution in turn introduces difficulties in separating fineparticles from relatively large volumes of gas, requiring elaboratefiltering systems. Consequently jet grinding or, in general, comminutiondepending on air currents, has been of limited use to reduce solidparticles to extremely small size, as for example sizes less than aboutmicrons.

Another problem evident in comminuting solids, even with gas jets or gasstreams, is the problem of heat. Heat develops when a particle isshattered by impact. The intensity of this heat may be considerable, atleast locally. With some substances avoidance of local temperatureincrease becomes critical. Many substances deteriorate or are destroyedby the heat developed locally in pulverization. A suitable example ispenicillin, for which some types of grinder are entirely unsuited andeven jet-type machines are inefficient to conserve antibiotic activity.Many substances other than pharmaceuticals are even more sensitive orlabile,- so that the heat problem is burdensome and in some cases iscritical.

In pulverizing, deteriorative effects result also from oxidation in someinstances, or local static electricity in others, or even frommechanical shock in others. Such efiects are difficul-t to avoid becausethey develop within the individual particles that are affected.

A broad object of this invention is to comminute small solid particleseffectively and with minimum deleterious effect, even to extremely fineparticles or the order of 30 microns or less. A subsidiary object is toaccomplish such pulverization that a large portion of these fineparticles are of any desired size and are of substantially uniform size.Another object is to produce these fine dusts, with relatively smallvolumes of gas, so that these dust particles can be readily recovered.

A further object is to obtain substantially homogeneous products inpulverizing solid particles of differing qualities. This is importantwith pharmaceuticals. For example, vitamin mixtures must be uniform sothat uniform dosages may be capsulated. Not only must segregation ofsome solids from other be avoided, but deterioration of someconstituents also must be prevented when rending to small sizes. Manycolloid mills are deficient in these respects. To obtain uniformproducts of ultra-fine solids, has formerly been extremely diflicult,even practically impossible without altering the activity of delicatecomponents.

Various other advantages of this invention will become apparent asexposition of this invention proceeds. It will be understood thatsuitable equivalents are contemplated in the illustrations and thedefinitions of the attached claims.

In the attached drawing illustrating a preferred embodiment of thisinvention, Figure 1 shows generalized apparatus diagrammatically,particularly including a smoothfaced, high-speed comminuting wheel 25.Figs. 2-6 inclusive show various illustrative forms of rotor peripheralface and annular operating zone. In these figures the operating zone isshown in fragmentary view somewhat enlarged relatively to the rest ofthe structure.

This invention applies extremely turbulent gas to solid particles totear the particles into very fine dusts. The term gas as used herein isintended to include vapors. Avoided are mechanical impact elements; evenimpacts of particles against each other are minimized.

The apparatus shown in the drawing illustrates various principles ofstructure and of operation under this invention. The apparatus isdesigned to produce extreme turbulence in a shallow comminuting zone ofgas 22. Means are provided to conduct small solid particles through thisviolently turbulent space for comminution, but with relatively littlevolume of gas. Many benefits of the invention are made possible bymoving a surface bounding the comminution zone at speeds of about 108 to700 feet per second to produce turbulence. This surface is preferablysmooth, but the arrangement diagrammed is effective to pulverize solidsto very fine dusts.

The preferred assembly of the apparatus illustrated in Figure 1,includes a tank 38 for supplying gas under pressure in communicationwith a feed valve l3, pressure control means 3|, and inlet conduits i2and Ill. Connected to the inlet conduits i2 and iii by means of flexiblehose in upper and lower portions 35, is the agitating chamber H providedwith an upper inlet hopper 32 having removable and scalable closuremeans 29. Chamber l i is mounted for reciprocal movement as on therollers St or other suitable support, and is agitated as by means of theeccentric mechanism 36 driven bysuitable motor not shown. The lowerportion of the agitating chamber I! is provided with an outlet hopper 33which is connected by the lower hose portion 35 to the inlet tube iii.Inlet tube if} communicates with the housing 23 of the grinder is at theinlet port 22 and is in communication through the zone 22 with theoutlet tube 83 which is connected to the housing 23 at the outlet port 43. The outer end of the outlet tube is is in sealed communication withthe trap chamber l4.

Although various dust recovery means may be used a very importantadvantage of this invention is that the dust produced, though ultrafine,may be recovered satisfactorily by simple trap 14 in cooperation withbag filter I when required. This advantage results from the exceedinglylow volume of gas required by the present invention to convey the solidmaterial through the mill and into the dust recovery apparatus.

Trap chamber H3 is joined by connection to the filter housing 38 and isin communication with the filter bag I5 by means of the conduit 31. Thefilter housing 38 is connected to the outlet tube 39 and the outletcontrol valve 5?.

The grinder comprises the housing 23 in which the wheel 24 is carried byits shaft 25 for high speed rotation in housing bearings 2E5 and 21.Shaft 25 is in operative connection with its pulley 28 which in turn isdriven by a main drive motor not shown. The wheel housing 23, and maindrive motor are designed to maintain wheel speeds at any desiredperipheral speed up to 700 feet per second or more.

The peripheral surface 2! of the wheel 24 is positioned so that there isa zone 22 between the housing 23 and the surface 25, and so that thereis a side clearance i6 between the sides of the wheel 24 and the sidesof the housing 23. The sides of the housing 23' may be tapered towardthe periphery of the wheel or they may be made substantially normal toshaft 25 as shown in Figure l. The whole unit ill may be submerged in atank 553 containing a temperature controlled environment 21 as apreferred means for maintaining predetermined temperature controlthroughout the operation of the unit til. Bath 4! is diagrammed inFigure 1 as surrounding pulley 23. This can be used for a gas bath butwhen bath 4%! is liquid, pulley 28 would preferably be arranged tooperate outside of tank 59.

Surface 2! is curved cylindrically, but may be curved otherwise. Shaft25 is supported massively in housing 239 and in high speed bearings 26and 27 arranged to be driven by direct con- 'nection from motor orturbine, not shown.

Speed of wheel 2% may also be stepped up, as by pulley 28 from a drivingpulley. Instead of a pulley drive any suitable, smooth-running drivingmeans adapted for high speed may be used.

Emphasis is placed on solid mounting and sturdy.

between 200 and 500 feet per second or about 0.03 inch for producingdust averaging below about 20 microns particle size when the peripheralspeed of wheel as is of the order of to 300 feet per second. A portionof the wheel face 2|, housing 23 and gas turbulence zone 22 is shownbroken away and enlarged for clarity. Spacing up to maximum of about0.05 inch is suitable.

The optimum of clearance depth of zone 22 depends upon the character ofthe material to be ground,'the degree of subdivision desired and therate of production desired. For any fixed clearance depth, and for agiven material the peripheral speed of the wheel 2 and/or the rate offeed of solid will determine the degree of subdivision obtained. Forexample if a material having low internal cohesion, such as graphite, isto be ground to medium fineness in which the particles will have anaverage diameter of 30 microns, the clearance depth of zone 22 may be aslarge as 0.05 inch, with the peripheral wheel speed of wheel 2 as low as100 feet per second,

and a feed rate of 50 lbs. of graphite per hour.

Finer subdivision will be obtained by increasing the wheel speed orreducing the feed rate, or both. Likewise, if it is desired to grind aharder material such as ypsum to a high degree of fineness such that theparticles would have an average diameter of 2 microns, eiiicientconditions would be a clearance depth of zone 22 of 0.015 inch, aperipheral wheel speed of 400 feet per second and a feed rate of 15 lbs.per hour. These conditions are dictated by the circumstances that theintensity of the turbulence produced in zone 22 is increased either byincrease in peripheral speed of wheel 24%, or by decrease in theclearance depth of zone 22, or both. Also, the length of time which agiven particle remains in zone 22 and hence is subjected to the actionof turbulence, depends upon the rate of feed. The longer this time, thesmaller the size of the comminuted particle and the more nearly uniformwill be the sizes of the particles. These intensit and time factors areindependently controllable and, therefore, high eiliciency ofcomminution of a wide variety of materials may be attained by adjustingthese controls in accordance withthe disclosures set forth.

Operation desired pressure and flow within the mill if], as

described herein. Main motor drive is adjusted to impart the desired R.P. M. Agitator means 36 and main motor drive to pulley 28 are energized.

' invention.

The particles 5| to be comminuted are introduced into chamber ll throughhopper 32, and cover 20 is sealed back in place and valves l and H arenext adjusted to deliver gas at the rate and at the pressure desired.Gas then passes through pipe 52, upper hose portion '35, and enterschamber El. Chamber ll is agitated sufficiently to cause particles tofall into hopper 33 at the desired rate. Particles 5i fiow with gas downthrough lower hopper at, lower hose portion 35, inlet conduit or ducti0, and inlet port 42 into the comminuting zone 22.

After comminution to the desired particle size, the gas stream carryingthe newly comminuted particles passes through outlet port 43, outletconduit or duct 13, into trap Hi which removes a major portion of thenewly comminuted particles 55, through tube 31 and into filter bag whichremoves the remainder of the newly comminuted particles from the gas,which is finally discharged through outlet tube 39 and exhaust valve l1.Subsequently, the motors are stopped, valve !8 closed, valve ii opened,covers 53 and 54 removed from filter chamber 38 and trap M.respectively. The comminuted particles 55 and 52 are removed from trap Mand from filter bag l5 respectively.

Example I A specific example will emphasize features of this inventionshown in Figure 1.

Cylindrical wheel 2 of high tensile strength, about 3.25 inches indiameter, was attached tightly to a steel shaft 25 of diameter. Thiswheel had a peripheral face 2i nearly one inch wide. The steel shaft wassturdily mounted in well-fitted and accurately aligned andwelllubricated high-speed bearings 26 and 2'! and connected to anair-driven turbine motor which rotated the wheel 24 at any desired speedup to 40,000 or more R. P. M. This is a relatively new order of speed inrotating surfaces. Such speed requires a high degree of care in designand of workmanship to avoid rupturing the machine itself. These speedsprovide many new and useful results.

The wheel 2:! and its shaft 25 and the housing 2'3 were made of steel inorder to obtain the requisite strength, but other materials may beutilized. For one example, wheel 24 may be composed of fibre glasscloth, laminated and resin-bonded to obtain a wheel having a high ratioof tensile strength to density. The peripheral face 2| is relativelysmooth. The width of this peripheral face 2! may bear a ratio todiameter of the wheel 213 from about 1 to 0.5 to about 1 to 6 for mostpracticable purposes.

The peripheral face 2i of the rotating member is technically smooth,from tool finished surface to lapped surface. Such surfaces accomplishunusual results at the high speeds utilized in this For example, face 2|of steel was brought first to a matte or satiny finish and comminutingeffects were studied of the resulting turbulence of air in space 22 atthe described ultra speeds of wheel face M. This was a satisfactory anduseful surface, for when solid particles were fed through the turbulencechamber 22 the particles were comminuted to an extremely fine dust. Thenface 2| was smoothed to an additional degree and coated withelectro-deposited chromium to produce a hard, non-corrosivesubstantially highly polished finish. Solid particles fed to zone 22when this surface was used were still comminuted to an equally finedust. Moreover, when the solid was very labile, such as penicillin,

the antibiotic activity of the resulting dust was equal to that of theinput material. Heretofore penicillin has lost in antibotic activity onbeing comminuted. Projections or roughness greater than about 0.01 inchdeep on the peripheral face 2| of wheel 24 tend to deteriorate thequality of such solids as penicillin because of the local heat effectsof impact at these tremendous speeds. Moreover, such roughnesses tend toclog and to unbalance the wheel. This is true with waxy substances suchas DDT insecticide. Consequently a surface of a high degree ofsmoothness is preferred for the moving face 2! of this apparatus. At theultra-speeds of this invention such a surface produces very highturbulence of gas in the peripheral clearance space 22.

The sides of wheel 24 and of housing 2'3 usually are straight to formstraight clearance between them. Impact may be minimized by taperingthis clearance slightly and by making side space I6 somewhat widerbetween walls than the peripheral clearance 22. The location and size ofports 42 and Z3 and conduits iii and it may vary. With the wheeldiscussed in the example, inlet port 42 and outlet port 43 were a halfinch in diameter, with their centers iocated inch below the peripheralface 22. One inlet port and one outlet port suihce, though variousnumbers and arrangements are useful also. Ports 02 and 43 were connectedrespectively to conduits l0 and 13 each of internal diameter. Outletduct is communicates with outlet valve i'i.

Example II As an example of processing very labile material under thisinvention, comminuticn of peni cillin is described. Feed chamber it wascharged with dry, granular penicillin of size passing a 20 mesh sieve. Asource of compressed, dry nitrogen was connected to gas inlet 52 throughvalve and pressure control indicator 33. Valve 58 was adjusted so thatpressure in conduit 62 was 3 p. s. i. g., and the gas flow was throttledto 0.5 cubic feet a minute by valve ill in the discharge line 30.Discharge line it was connected to a conventional trap and bag filter.The wheel 24 which was 3.25 inches in diameter and 1 inch wide wasbrought to a peripheral speed of about 500 feet per second,corresponding to 35,000 R. P. M. The feed chamber H was vibrated tocause penicillin to flow into the grinder it at the rate of about 20grams a minute. The comminuted product was collected readily in the trapl4 and bag filter E5. The output product was as active antibiotically asthe input material. The mass median diameter of the particles producedwas 6.3 microns in this example, while the frequency distribution ofdiameters was such that percent of the particles were of diameters lessthan 4 microns.

Certain observations may emphasize the important advance in the artresulting from this novel pulverization by turbulence of gas at asurface moving at ultra speeds in a comminution. chamber as illustratedat 22 and described herein.

First, only relatively little gas flows through the machine. This flowneed by only sufficient to carry the particles through the machine andthe assembly described. For example, penicillin comminution requiredonly 0.5 cubic feet minute of gas through-put, instead of the order ofcubic feet per minute of gas for comminution by gas jet. Since thisvolume of gas is small, difficulties of recovering fine dust areminimized. Those familiar with bag filters or with collec- 7 tion ofdusts will appreciate the importance of being able to use a bag filterat all in this case, or of being able to substitute other collectinmeans if desired.

Another advantage of the small amount of gas deserves considerableemphasis. The flow of the small amount of gas can contribute onlyinsignificantly to the turbulence of gas in the machine. The work doneon the suspended material being comminuted is governed by the mechanicalenergy supplied to the rotating member which produces the requiredturbulence. This ener y is readily controlled. Therefore, a high degreeof control of the processing is readily obtained. This avoids the lackof control in attempting to obtain gas turbulence by release of largequantities of costly compressed gas.

A further advantage of small through-put of gas is that it permitsmaterials containing one or more volatile constituents to be processedwithout significant loss of volatile matter. Different batches of suchmaterials can be blended or be comminuted together to obtain a productof uniform composition, while economic loss of Volatile constituentscarried away by exhaust gas is minimized.

The small volume of gas through-put permits even costly gases to be usedas the atmosphere in which the solid particles are carried through themachine. Thus materials heretofore prohibitive become economical. Forexample, use of nitrogen in small quantity makes feasible thecomminution of protein material such as egg albumen that woulddeteriorate in air. The finer the particle is pulverized, the moresurface is bared and the greater the oxidation that would occur in air.Furthermore, with materials that require dry gas, or any given humidity,or require pure oxygen, or can utilize any special gas, the novel lowgas through-put makes feasible the use or" such gas in the presentinvention.

An additional further advantage is that the pressure at whichcomminution occurs can be varied readily or can be held at any desiredlevel, simplj by adjusting valves is and 57. Or similarly, pressure lessthan atmospheric can be obtained by reducing or varying the flow fcompressed gas from tank 353 by means of inlet gas valve 18 andexhausting gas through the outlet ll. Combinations or alterations orthese adjustments of valves I! and i8 produce suitable difierentialpressures within the pressure range of subatmospheric tosupenatmospheric.

Another advantage of this invention lies in its ease of temperaturecontrol. When the appa ratus is placed in a thermally controlledenvironment, such as in a gas bath or a liquid bath H, conduction ofheat occurs readily through the housing that confines the working area.Since the working zone 22 is of small cross-section such heat controlXtends readily to the particles being comminuted therein. Moreover, thiscontrol is both sensitive and substantially complete because of theextremely high turbulence in the working area. Predeterminedtemperatures of the particles being comminuted thus are readilymaintained.

To protect individual solid particle against local heating from chanceimpact along the side of the wheel 2 1, side clearance it greater thanthat of the intercommunicating peripheral comminution zone 22 avoidsdrastic throw or sweep of particles outwardly. This also minimizesimpacts against the housing 23 and augments the effect of locating aparticle inlet port 42 near the periphery 2! to further reduce thisthrow.

The present invention also permits the compressibility of the fluid toimpede the outward throw of the particles. This cushioning action alsoreduces mechanical shock and local heating of particles. Centrifugaldensification or" gas outwardly tends to minimize movement of the solidmaterial except in the narrow zone of turbulence at the super-speedsurface 2 l. Consequently reduction to super-fine colloidal dust issubstantially confined to the forces of the turbulent gas itself.

Comminution occurs in the space between the rotor periphery 2i and thehousing 23 by action of the turbulent gas on the individual solidparticles. To be distinguished from impact of particle against particle,is this rending apart by forces of gas acting either in shear or intension. Such rending apart is evidenced by the fact that with thisinvention the lower the ratio of solids to gas the finer thecomininution obtained. The entire arrangement described makes possiblesubstantially isothermal conditions in reducing solid particles to dust.

It becomes apparent from the foregoing that the present inventionprovides independent regulation of turbulence, temperature and pressurewithin the turbulent comminution zone 22, as well as of particle feed.

The effects of static electricity are also avoided. The intimate contactof processed solids with electrically conducting parts of the machineprovides adequate conduction to eliminate accumulation of staticelectricity. Not only does this reduce hazards of explosion, butfacilitates separation of such fine particles from the eiiluent. Mutualrepulsion, and consequent suspension, oi dust particles heretoforeworked against collecting super-inc, colloidal dusts.

Size of particles fed to this apparatus varies with conditions andmaterials. In general, feed size of the order of 103 microns or less isdesirable, but even as large as 20 mesh particle feed is suitable withsome materials. Particles are readreduced to less than 2! micronsdiameter by this invention.

Though this invention is not limited by theory, it appears from researchthat the speed of the moving peripheral surface at and the narrowcross-section of the working space 22 produce exceedingly highturbulence of gas. It also appears th'at the many high velocitygradients, corresponding to high energy gradients, subject a particle tomany intense and changing stresses in shear and intension. When thestresses exseed the cohesiveforce within the particle, the particlesubdivides. Evidently at comminuting speeds of this apparatus, the gasin the peripheral working space is not simply carried along with themoving surface but breaks up into local and individual rotating massesor rollers. These spin on their individual axes of rotation at muchhigher rates than even the wheel surface, dependent on their owneffective diameters relatively to the rotated wheel, as with gears.Solid particles caught in the spin of these whirling gases rotate attremendous angular rates to develop disrupting centrifugal forces.

The rate of rotation of the wheel 25, determining the degree ofturbulence of the gas and consequently the particle size obtained,varies with.

clliierent peripheral clearances. Small clearance andhigh speedsubdivide solid particles most eirectively. Rate of flow and driftthrough the apparatus 50 also affect particle size and sizeuniformly ofproduct since lowering the feed-rate increases duration of time toobtain small pai ticle production. In general, peripheral surface speedsof the wheel above one hundred feet per second are suitable forcomminuting solids to very fine particles. Peripheral clearance may befrom about 0.01 inch (0.25 millimeter or 250 microns) to about 0.05inch. In Example I, where the wheel was 3.25 inches in diameter andabout one inch wide at its peripheral face, a feed rate of about 20grams a minute of solid penicillin suspended in. gas flowing about 0.5to two cubic feet of gas a minute was about optimum. These conditions ofcourse vary with different materials and sizes. Too fast a feed clogsthe machine and this clogging point is readily observed. The apparatusof Example I showed a rather sharp beginning of cornminuting char'acteristics at about 12,000 R. P. M. (or a peripheral speed of 1'70feet per second) increasing rapidly to about 13,000 R. P. M., andincreasing slowly thereafter to 42,000 R. P. M.

At the speeds used in this invention, it is essential that all details,workmanship and design be of highest quality so as to avoid rupture inthe machine from the centrifugal forces involved. Stresses may developthat exceed strengths of known materials, particularly if unbalancedparts are rotated at these speeds. Projections of various sorts tend toserious unbalance, for the order of magnitude of these speeds andstresses far exceeds those of prior comminuting machines. Consequently,the smooth,

satiny-surfaced cylindrical wheel as described herein is required toobtain the necessary high speed surface.

The apparatus of the present invention has been used successfully forcomminuting a variety of solids. From this use it has been found thatthe processing of these materials has fallen within the followingpreferred ranges: in the comminuting zone 22, a pressure range of from.5 to atmospheres absolute pressure was found desirable; the bath orenvironment zone at ii was maintained at temperature ranging from C. to+80 C.; the particle feed delivered by agitator 36 to hopper (-33 ispreferred at rates ranging from one gram of solid particles per minuteto 30 and more pounds per hour; the rate of feed for gas from source asto port i2 is preferred at a range of from .2 to cubic feet per minute;

the depth or thickness of zone 22 is preferred not to exceed of an inch,the optimum thickness being from .01 to .08 of an inch; the pcriferalface 2! is preferably provided with a width of from about to two timesthe diameter of wheel 2%; and the periferal speed of wheel 2 ispreferred within the range of from 100 to 700 feet pcrsecond.

Comminution at temperatures well below freeaing increases thebrittleness of certain solid and thereby increases the rate at whichthey may be subdivided. Other solids, however, are comminuted moreemciently in environments maintained at the above-mentioned highertempera ture ranges. Environments excessively dry or excessively humidare also sometimes desirable for the oomzninution of certain solids.Various ga. es or mixtures of gases have also been found desirable forother solids. All solids are easily and quickly recovered aftercomminution by the simple recovery means exemplified at M and The depthor thickness of zone 2?. may be 1naintained at predetermined distancesby varying the diameter of wheel E l; or by inserting thin liners ofsheet material in zone 22 on the underside of housing 23 adjacent tosurface 2!; or by the use of a conical surfaced wheel in place ofcylindrical surface 2|. This conical wheel is adjustable along its axisand operates in a conical zone rather than in zone 22, as shown.Combinations of these means, as well as other means of regulation, mayalso be used if desired.

This invention and the manner of making and using it is hereindescribedin full, clear, concise and exact terms so that those skilled in the artmay make and use it. Its principle and preferred mode of applicationhave been explained. However, it is intended that this inventionincludes all modifications and embodiments within the spirit and scopeof the appended claims.

1. The method of comi nuting finely divided particles into substantiasmaller particles, comprising agitating finely divided particles in agas under compression, thereby suspending said particles insaid gas,passing said gas with said particles in suspension through a narrowannular turbulent having a width not in excess of 1 of an inch and beingbounded on its outside by a rigic stationary surface and on its insideby a hard smooth surface moving at a speed of not less than feet persecond.

2. A process for reducing solid particles to fine dust comprisingadvancing in a narrow zone gas containing solid particles, confining thezone between two wall not more than 0.12 inch apart, causing relativemovement between the two walls one to the other of at least 100 feet persecond, thereby causing high turbulence close to a wall surface andpulverizing the solid particles carried in the gas to fine dust of theorder of less than ten microns.

3. A process for reducing solid particles to fine dust comprisingadvancing in a narrow zone gas containing solid particles, confining thezone between two Walls about 0.01 to 0.12 inch apart, causing relativemovement between the two walls one to the other of about 300 feet persecond, thereb causing high turbulence close to a wall surface andpulverizing the solid particles carried in the gas to fine dust of theorder of less than ten microns.

4. A. process for reducing solid particles to fine dust comprisingadvancin in a narrow zone gas containing solid particles, confining thezone as an annulus between twoarcuate concentric walls about 0.01 to0.12 inch apart, causing relative rotative movement between the twowalls one to the other transversely of the general direction of the gastravel and narrow zone of at least 100 feet per second, thereby causinghigh turbulence close to a wall surface and pulverizing the solidparticles carried in the gas to fine dust of the order of less than tenmicrons.

5. Apparatus for comniinuting solid particles comprising in combinationa casing containing a substantially cylindrical recess, a substantiallycylindrical rotor mounted in said casing and provided with a smoothcircumferential surface arranged for rotary movement in said recess anddisposed in" the recess to form with the casing an operating annularperipheral zone clear along the smooth rotor surface and having uniformradial depth to give clearance of the order of 0.01 to 0.12 inch,driving means arranged to provide said movement at a peripheral speed ofat least 100 feet per second, there being a gaseous atmosphere in theperipheral operating zone, a compressed gas supply and conduittherefrom, and means for mixing solid particles in suspension in gas inthe conduit, said conduit having an opening for feeding gas and.particles into the recess in close proximity to said peripheraloperating zone, and outlet means at the opposite side of the rotor inclose proximity to said peripheral zone.

6. Apparatus for comminuting solid particles comprising in combination acasing containing a recess, a substantially cylindrical rotor mounted inaid casing and provided with a highly polished circumferential surfacearranged for rotary movement in said recess and disposed in the recessto form with the casing an operating annular peripheral zone clear alongthe polished rotor surface and having uniform radial depth to giveclearance of the order of 0.01 to 0.12 inch, driving means arranged toprovide said movement at a peripheral speed. of at least 100 feet persecond,

there being a gaseous atmosphere in the peripheral operating zone, acompressed gas supply and conduit therefrom, and means for mixing solidparticles in suspension in gas in the conduit, said conduit having anopening for feeding gas and particles into the recess in close proximityto said peripheral operating zone and outlet means at the opposite sideof the rotor in close proximity to said peripheral zone.

7. Apparatus for reducing solid particles to fine dust comprising incombination a casing con taining a recess, a rotor mounted centrallytherein, bearings at each end of the rotor and means to drive the rotor,all adapted and being of size and material and. being balanceddynamically for rotor peripheral speeds of at least 100 feet per second,the rotor having a circular and smooth peripheral face, the casingrecess and the rotor peripheral face conforming to each other anddefining a clear annular operating zone between rotor and easing havinguniform radial depth to give about 0.01 to 0.12 inch clearance along theperipheral face, there being a gaseous atmosphere in the peripheraloperating zone, a source 12 r of supply of gas, a conduit therefromhaving an opening into the casing recess at one side of the rotor inclose proximity to the annular operating zone, means to feed gas fromthe conduit into the annular operating zone, means to feed solid par-REFERENCES CITED The following references are of record in the file ofthis patent: V

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